Temperature And Energy Di St Ri Hut

Fig. 7.2 shows a typical temperature distribution in a Stirling engine of advanced design. Inlet-air enters the engine at the atmosphere temperature and is heated in the preheater before passing to the combustion space. Fuel is added and combustion occurs, heating the products of combustion to a very high temperature. The combustion products then pass through the heater, where heat is transferred to the working fluid, and through the preheater. where heat is transferred to the inlet-air. The cooled products finally leave the engine. In many applications where air pollution is an important consideration, a fraction (up to one half) of the exhaust products arc recirculated back through the combustion chamber. This increased mass flow of relatively inert fluid moderates the maximum temperature attained in combustion and so reduces the amount of oxides of nitrogen (NOJ produced. Some extra work is then required to cause the air to flow through the system and therefore a fan will be necessary.

This will most likely be found on the 'cold' inlet-air side. The combustion chamber and preheater therefore operate at a pressure slightly above atmospheric. A fuel pump will also be required to supply and. perhaps, atomize the fuel.

Working fluid in the engine passing from the regenerator to the expansion space is heated in the heater by energy transferred from the products of combustion. 1'he working fluid, probably hydrogen or helium at high pressure, has excellent heat-transfer characteristics compared with the combustion products at atmospheric pressure. For this reason a very high temperature difference will most likely exist between the temperature of the combustion products and the heater-tube walls compared with the temperature difference between the tube walls and working gas.

At lower temperatures heat is transferred from the working fluid in the cooler near the compression space. The heat is rejected from the cooler to water circulated through the engine. Water is a dense fluid with excellent heat-transfer properties and so only a small temperature deference in the cooler is required to elfect the energy flow.

The heat rejected from the working fluid to the cooler is eventually dissipated to the atmosphere in an air-cooled radiator. A fan is required to draw air through the radiator and a pump is necessary to circulate the water.

In stationary power applications the water may be drawn from a virtually infinite source such as a lake or river. Even here, however, an intermediate cooling system will usually be preferred to avoid contamination of the engine cooler by sediments drawn in by the cooling stream.

Energy flows in a Stirling engine are shown in Fig. 7.3. This Sankey diagram is reproduced from the important and fundamental paper by Zacharias (1971a). Starting front the top of the diagram there is a given energy input to the system (designated as 100 per cent). Recirculation of a fraction of the exhaust gas is shown to an energy equivalent of about 43 per cent. Power and other losses for recirculation consumes about three per cent of the input energy supply.

For the particular case shown the energy loss from the system carried off in the heated exhaust gas is about 14 per cent.

At the bottom of the diagram the heat loss to the cooling water is 45 per cent, mechanical friction consumes a further 5 per cent anil approximately 32 per cent of the energy supplied is transformed to work available at the engine shaft. An important point to note from Fig. 7.3 is the very high levels of energy flow in the regenerator, amounting to over four times the energy input to the heater, or alternatively nine times the energy flow in the cooler, and twelve times the energy flow to work. The 'recycling' of this remarkable energy flow is the significant attraction of the Stirling engine.

1 hernial cncryv input (100%)

1 hernial cncryv input (100%)

Fjc. 7.3. Energy How in a Stirling engine

Fjc. 7.3. Energy How in a Stirling engine

Ivig. 7.3 illustrates well the important differences between an internal combustion engine and a Stirling engine. To a first approximation the energy flows in a diescl engine may be thought of as three equal streams comprising 33 per cent each to work, to cooling, and to exhaust. Thus the energy How to exhaust is twice that of the Stirling engine and the cooling system load is much less.

In a dicsel engine, a high energy flow to exhaust is permissible because combustion of the fuel occurs inside the engine cylinder, so that no heater is necessary and the heal not converted to work must be dissipated either to exhaust or the cooling system. This is not the ease in the Stirling engine where combustion of the fuel occurs outside the engine cylinder. Any heal passing to the exhaust represents a direct loss of energy that has through the heater and not converted to work must be dissipated by the cooling system.

Returning therefore to the example given by Zacharias and reproduced in Fig. 7.3, every etfort should be directed to reducing the exhaust stack loss from 15 to HI or even 5 per cent. This would most likely increase the fraction converted to work from 32 to 35 per cent leaving 55 to 60 per cent to be handled by the cooling system and by extraneous convective and radiative heat transfers from the engine. This heavy load in the cooler and the sensitivity of the engine efficiency to increase in the cooler operating temperature is one of the major stumbling blocks to the use of Stirling engines in automotive applications.

The solar Stirling engine is progressively becoming a viable alternative to solar panels for its higher efficiency. Stirling engines might be the best way to harvest the power provided by the sun. This is an easy-to-understand explanation of how Stirling engines work, the different types, and why they are more efficient than steam engines.